Computed Tomographic Assessment before Transcatheter Aortic and Mitral Valve Replacement

Mona Bhatia; Parveen Kumar; Prasit Maity; Natisha Arora

doi:10.4103/jiae.jiae_38_22

REVIEW ARTICLE

Year : 2022 | Volume : 6 | Issue : 3 | Page : 248-254

Computed Tomographic Assessment before Transcatheter Aortic and Mitral Valve Replacement

Mona Bhatia, Parveen Kumar, Prasit Maity, Natisha Arora
Department of Imaging, Fortis Escort Heart Institute, New Delhi, India

Date of Submission	05-Jul-2022
Date of Acceptance	11-Jul-2022
Date of Web Publication	12-Nov-2022

Correspondence Address:
Dr. Mona Bhatia
Department of Imaging, Fortis Escort Heart Institute, New Delhi
India

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jiae.jiae_38_22

Abstract

Transcatheter aortic valve replacement (TAVR) and transcatheter mitral valve replacement (TMVR) are catheter-based interventional techniques for treating patients having high risk for surgical aortic or mitral valve replacement, respectively. While TAVR is a technique for treating severe aortic stenosis, TMVR is primarily used for treating mitral regurgitation. Echocardiography is the initial imaging modality used for a detailed assessment of the mitral and aortic valve lesions. Multidetector computed tomography (MDCT) is then used as a complementary tool to provide additional information essential for pre-procedure planning. High spatial resolution and good temporal resolution of MDCT along with multiplanar reconstruction technique permit a comprehensive assessment relevant for the multiple aspects of preprocedural planning. The current article outlines the MDCT acquisition protocols, reconstruction techniques, and assessment of various essential parameters for TAVR and TMVR.

Keywords: Cardiac imaging, neo-left ventricular outflow tract, structural heart disease interventions

How to cite this article:
Bhatia M, Kumar P, Maity P, Arora N. Computed Tomographic Assessment before Transcatheter Aortic and Mitral Valve Replacement. J Indian Acad Echocardiogr Cardiovasc Imaging 2022;6:248-54

How to cite this URL:
Bhatia M, Kumar P, Maity P, Arora N. Computed Tomographic Assessment before Transcatheter Aortic and Mitral Valve Replacement. J Indian Acad Echocardiogr Cardiovasc Imaging [serial online] 2022 [cited 2023 Sep 27];6:248-54. Available from: https://jiaecho.org/text.asp?2022/6/3/248/361062

Introduction and Procedural Overview of Transcatheter Aortic Valve Replacement

The most effective treatment for severe aortic stenosis (AS) is surgical aortic valve replacement (SAVR). However, a large number of patients are ineligible for this curative treatment due to high perioperative risk.^[1],[2] TAVR (or transcatheter aortic valve implantation [TAVI]) is a new procedural option, in which the prosthetic valve is transported to the aortic root using endovascular approach. TAVR was first introduced in 2002, and it has become an accepted and widely validated option for patients considered at high surgical risk.^[3],[4] Preoperative noninvasive imaging is of paramount importance for annular sizing, determining risk of annular injury and coronary occlusion, and providing coplanar fluoroscopic angle. MDCT plays an essential role in the preprocedural assessment. While computed tomography (CT) was initially used mainly for the assessment of peripheral vascular access, its role has grown substantially and now it is the gold standard imaging tool for patient and device selection.^[5]

Computed tomography acquisition protocol

The CT scanning protocol involves two separate acquisitions: (i) a retrospective electrocardiography (ECG)-gated scan for aortic root and (ii) a large coverage nongated acquisition from subclavian to femoral arteries.

Images should be reconstructed at 1.0 mm or less for good-quality multiplanar reformations (MPR), which necessitates at least a 64-slice or dual-source scanner. According to the Society of Cardiovascular Computed Tomography recommendations, a tube potential of 100 kV should be considered for patients weighing ≤90 kg or with a body mass index (BMI) ≤30 kg/m²; whereas a tube potential of 120 kV is usually indicated for patients weighing >90 kg and with a BMI >30.^[5] The scan is triggered using bolus tracking technique by using a small region of interest in the ascending aorta or the left ventricle. The data acquisition begins when the attenuation in the region of interest reaches a threshold of 150 HU. A single injection of contrast agent is recommended for both acquisitions. Around 50-ml contrast material at a flow rate of 3–4 ml/s is usually sufficient, but it may have to be modified according to the patient's body habitus and the CT scanner capabilities.^[6] Beta-blockers and nitrates are contraindicated in severe AS and should not be administered before scanning. The retrospective ECG-gated scan provides motion free assessment of the left ventricular outflow tract (LVOT), aortic annulus (AA), sinotubular junction (STJ), coronary ostia, and ascending aorta. A noncontrast gated scan is usually done to calculate the calcium score of the aortic valve. The large nongated second acquisition extending from subclavian arteries to the superficial femoral arteries is adequate to provide the assessment of aortofemoral vessels.^[7]

Assessment of aortic annulus

The aortic valve complex consists of AA, sinuses of Valsalva (SOV), coronary ostia, and STJ [Figure 1].^[8],[9] The AA is oval shaped and undergoes conformational changes during various phases of cardiac cycle. The annular anatomy should be carefully reviewed to obtain the largest dimension with adequate image quality, and the systolic measurements are preferred for the measurement and calculation of device sizing. The double-oblique MPR images provide the correct assessment of annulus dimensions. The commonly used measurements for the appropriate annular sizing and prosthesis selection include long-axis and short-axis diameters of the oval AA, area-derived annulus diameter, and perimeter-derived annulus diameter.

Figure 1: Contrast-enhanced retrospectively ECG-gated images of aortic valve complex in a prospective TAVR candidate. (a) Coronal contrast-enhanced CT image demonstrates the levels of the LVOT (red arrow), annular ring (green line), sinus of Valsalva (yellow line), sinotubular junction (blue line), and ascending aorta (purple arrow). (b) Axial contrast-enhanced CT image demonstrates the LVOT. (c) Axial contrast-enhanced CT image demonstrates the ellipsoid shape of aortic annular ring. (d) Axial contrast-enhanced CT image demonstrates the cloverleaf shape of the sinuses of Valsalva. (e) Axial contrast-enhanced CT image demonstrates the true circle shape of sinotubular junction. (f) Axial contrast-enhanced CT image demonstrates the true circle shape of ascending aorta. CT: Computed tomography, ECG: Electrocardiography, LVOT: Left ventricular outflow tract, TAVR: Transcatheter aortic valve replacement

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Landing zone calcification

The landing zone comprises LVOT, AA, and valve cusps. Studies have demonstrated that severe LVOT calcifications are associated with increased risk of paravalvular regurgitations.^[10-12] In addition, the large LVOT calcification may cause annular rupture, particularly with balloon expandable valves. Furthermore, the landing zone is in close relation with the conduction system, and any compression caused by prosthetic valve on calcified LVOT may lead to conduction block.^[13],[14] Therefore, special attention should be given to landing zone calcification. In clinical practice, the LVOT calcification is reported in a subjective manner and is graded from none, mild, moderate, and severe types.

Aortic valve morphology

The most common morphology of aortic valve is tricuspid. Bicuspid aortic valve (BAV) is found in 6% of the patients presenting for TAVI. BAV is associated with lower device success rates and higher rates of paravalvular regurgitation.^[15] The commonly used classification for BAV is Sievers classification, which distinguishes the BAV morphology depending upon the number of raphe and commissures.^[16] It is difficult to define the annulus in BAV, specially Type 0, where there are only two hinge points. Annular size should be measured in the similar manner like for tricuspid valve. CT report must include the raphe characteristics such as raphe length and raphe calcification. BAV is also associated with ascending aortic aneurysm. Therefore, ascending aorta should also be examined more carefully in BAV.^[17],[18]

Sinus of Valsalva and coronary ostial height

SOV diameter is measured from the cusp to commissure, parallel to the annular plane. The three values of the SOV measurements can be averaged. There is no recommendation whether it should be performed in diastole or systole. Assessment of coronary arteries is another important step in pre-TAVR CT evaluation [Figure 2]. Coronary occlusion is a rare, but very feared complication of TAVR. It has been seen that a low coronary ostial height (<12 mm) along with small SOV diameter (<30 mm) increases the risk of coronary occlusion.^[19],[20]

Figure 2: Contrast-enhanced retrospectively ECG-gated images of aortic valve complex in a prospective TAVR candidate (a) Double-oblique contrast-enhanced CT image showing assessment of the distance of right coronary ostium from the aortic annulus plane.(b) Double-oblique contrast-enhanced CT image showing assessment of the distance of left coronary ostium from the simultaneously imaged aortic annulus plane. CT: Computed tomography, ECG: Electrocardiography, TAVR: Transcatheter aortic valve replacement

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Sinotubular junction

The height of STJ is another relevant factor in the preoperative decision-making for TAVR. If the height of STJ is low, the prosthetic valve may get into contact with the STJ, especially with balloon expandable devices. The STJ height should be measured perpendicular to annular plane. The diameter of STJ is also important and should be compared with the prosthetic valve size as small STJ diameter can increase the risk of STJ injury.

Aorto-ventricular angle

The aorto-ventricular angle is the angle between the proximal aorta, AA, and LVOT and is an important consideration when using longer prosthetic valves that require deployment perpendicular to the native annular plane [Figure 3]. Each type of transcatheter prosthetic valve has a maximum aorto-ventricular angle for successful valve deployment, typically 70° for the iliofemoral approach and 30° for the subclavian approach (due to angulation from the right subclavian into the ascending aorta).

Figure 3: Contrast-enhanced retrospectively ECG-gated image showing the aorto-ventricular angle. ECG: Electrocardiography

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Assessment of the access route

The most commonly used access route for TAVR is iliofemoral axis. CT provides accurate assessment of the vascular luminal diameter, tortuosity, and atherosclerotic plaque burden. The minimal diameter of the vasculature between the aortic valve and the right and left common femoral artery should be reported using a double-oblique MPR technique [Figure 4]. The extent and distribution of iliofemoral vasculature calcifications should be assessed and graded in a subjective, semi-quantitative scale: none, mild, moderate, severe. Care should be taken to identify the horseshoe-shaped near-circumferential calcifications mainly in areas of tortuosity or bifurcations, as these prevent vessel expansion while passing the sheath and the valve passes through. Tortuosity of vascular structures is another important finding, which should be evaluated using volume-rendered display. In the absence of vascular calcification, the tortuosity is not a contraindication to TAVR procedure; however, the calcified tortuous segments increase the risk of access failure. In addition, the report should include all the other vascular pathologies such as aneurysm, occlusion, and dissection, if present.^[21],[22] If the transfemoral access is not feasible, the subclavian or carotid arteries should be reported using the same technique and parameter as for femoral arteries. Left subclavian artery is usually preferred due to favorable angulation.^[23],[24] Finally, if the inferior vena cava is used for the access, the presence, size, and level of calcification-free windows should be reported.^[25]

Figure 4: Image showing three-dimensional volume-rendered computed tomographic reconstruction (right panel), and corresponding contrast-enhanced straightened view (middle panel) demonstrating aorta and right iliac arteries. The left panel shows contrast-enhanced true axial images used for measuring minimal luminal diameters at various levels

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Incidental findings

TAVR candidates are invariably advanced aged and may have significant incidental findings that may alter the treatment course. MDCT, with large anatomic coverage, is an ideal screening tool to identify significant incidental disease in up to 34.3% of cases and unsuspected malignancy in 4.1%.^[26] However, it is important to consider the dismal prognosis of untreated symptomatic AS, with short-term survival rates even lower than those of some undetected malignancies. Therefore, treatment options are always reviewed on a case-by-case basis.

Transcatheter aortic valve replacement in challenging cases

Bicuspid aortic valve

BAV is the most common congenital cardiac anomaly in the general population. The restricted leaflet mobility and progressive calcification contribute to the development of AS in these patients. TAVR may be used as an alternative to surgery in patients with BAV stenosis who are poorly suited for SAVR. However, compared with patients with tricuspid AS, higher rates of residual aortic regurgitation, mortality, and need for second valve have been reported. Factors that contribute to suboptimal TAVR outcomes are asymmetry of aortic valve cusps, annular eccentricity, calcification of the LVOT, fused calcified raphe, aortopathy, and aortic root dilation.^[27]

Valve-in-valve implantation

Valve-in-valve (VIV) implantation has evolved as a treatment alternative for failing bioprosthetic valves. There are higher chances of coronary occlusion after VIV than TAVR in native valve, with a reported incidence of 2.3% in comparison to 0.66% for native valves.^[28] Preprocedural CT plays an important role in the sizing and identification of high-risk candidates.

Transcatheter aortic valve replacement in transcatheter aortic valve replacement

TAVR-in-TAVR placement is used to treat acute TAVR failure in 1.4%–6.7% of patients.^[29-32] The final outcome of this procedure has been reported in a few small series. MDCT is useful in assessing the size of TAVR valve using the internal diameter of the prosthesis at the level of the leaflet insertion. The risk of coronary ostial occlusion is high, and special attention is given to the height of the STJ, which, if exceeded by the height of the prosthesis, has the potential to become sealed.^[33]

Introduction and Procedural Overview Of Transcatheter Mitral Valve Replacement

Mitral valve diseases include a complex set of conditions affecting the mitral valve. Mitral regurgitation (MR) is the most common mitral valve disease,^[34] commonly classified into primary MR (caused by intrinsic abnormality of mitral valve apparatus) and secondary MR (caused by incomplete leaflets coaptation due to annular dilation or leaflet tethering). While medical management plays an important role in the management of MR, surgical repair or replacement is often needed when MR is severe and adversely impacting cardiac hemodynamics.^[35] However, many patients are high-risk candidates for surgery and cannot undergo mitral valve surgery. Transcatheter mitral valve repair by using the MitraClip device is an alternative treatment option for MR in high surgical-risk candidates.^[36],[37] However, not all the candidates have favorable anatomy for MitraClip repair. TMVR is a novel interventional therapy for patients with severe MR who are poor surgical candidates and have unfavorable anatomy for MitraClip device.^[38] Herein, we review the role of CT in preprocedural assessment of TMVR.

Computed tomography acquisition protocol

A high-quality examination for TMVR requires at least 64-slice MDCT. Preprocedural medication is not required. Retrospective ECG gating is needed as mitral valve apparatus and LVOT undergo dynamic changes throughout the cardiac cycle.^[39] A triphasic contrast administration protocol is selected to opacify both right and left heart chambers. Scan is triggered by bolus tracking by using small region of interest in the ascending aorta. The scan coverage extends from carina to diaphragm, and the field of view includes the adjacent chest wall. The data acquisition is done throughout the cardiac cycle. The radiation dose is minimized by using the lowest possible tube voltage and current, as well as iterative reconstruction.

Assessment of mitral valve apparatus

It is imperative to select the appropriate-sized TMVR valve, as undersizing will lead to device mobilization and oversizing can result into mitral annular rupture. The mitral annulus has a complex saddle shape with anterior horn extending between trigones.^[40] A simplified D-shaped annulus is derived by eliminating the anterior horn and joining the trigones through a virtual line. It has been seen that simplified D-shaped annulus is more reproducible for device sizing. Most of the commercially available software involve manually placing the seeds points along the insertion of the mitral valve along 360° perimeter. The software then creates a three-dimensional perimeter of the saddle-shaped annulus. The two-dimensional mitral annulus can be generated manually by manipulating the MPR images. The most commonly used measurement is the perimeter. The linear distance between medial and lateral trigones is called trigone–trigone (TT) distance. The maximum diameter of the annulus parallel to the TT distance is called intercommissural distance. The septum to lateral distance is the maximum annulus diameter perpendicular to the intercommissural distance [Figure 5].^[41]

Figure 5: (a) Schematic diagram representing the mitral annulus. (b) Contrast-enhanced retrospectively ECG-gated image of LV short-axis of the mitral annulus plane showing the 2D saddle-shaped annulus. The TT distance is the linear distance between the trigones. The maximum annulus diameter parallel to the trigone-trigone distance is the IC distance. The maximum annulus diameter perpendicular to the IC distance is the SL distance. 2D: Two-dimensional, ECG: Electrocardiography, IC: Intercommissural, LT: Lateral trigone, LV: Left ventricle, MT: Medial trigone, SL: Septolateral, TT: Trigone–trigone

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Estimating the risk of left ventricular outflow tract obstruction

The placement of TMVR device causes anterior deviation of the anterior mitral leaflet which in turn causes narrowing of the LVOT, referred to as neo-LVOT. Obstruction in the neo-LVOT is a common cause of mortality and morbidity after TMVR procedure. CT can simulate the virtual prosthesis, mimicking the expected position. Following the virtual placement of the device, a long-axis view of the LVOT is generated in end-systolic phase. Using this view, a curved MPR image is generated which can provide the shortest axial cross-sectional area corresponding to the smallest diameter of neo-LVOT [Figure 6]. Although there is no cutoff, the studies have shown that the ideal candidate must have a neo-LVOT area more than 200 mm².^[42-44]

Figure 6: Virtual mitral device simulation, and neo-LVOT cross-sectional area measurement. Contrast-enhanced retrospectively ECG-gated image showing neo-LVOT. Dedicated software is used for calculating neo-LVOT area after simulation of a TMVR prosthesis implantation. Deflection of the anterior mitral leaflet into the LVOT results in elongation of the LVOT and formation of the neo-LVOT. A centerline is extended along the long-axis of the neo-LVOT and then cross-sectional evaluation permits measurement of the neo-LVOT area. ECG: Electrocardiography, LVOT: Left ventricular outflow tract, TMVR: Transcatheter mitral valve replacement

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Characterization of landing zone

CT is an important tool for the assessment of landing zone. Typically, a landing zone with 20% atrial offset and 80% ventricular offset is used for TMVR devices. Multiple factors including atrioventricular shelf, mitral annular calcification (MAC), and mitral annular disjunction affect the decision of device placement. CT provides detailed assessment of the MAC including the calcium extent, severity, and density. The Sapien valve requires extensive MAC, involving at least 75% of the annulus circumference, while for other devices, MAC is considered a contraindication in early studies.^[45],[46] An atrioventricular shelf is seen in patients with functional MR owing to basal myocardial remodeling. A persistent shelf is relevant for Tiara device that uses the basal myocardium for anchoring. In patients with mitral annular disjunction, the measurements should be taken at expected landing zone instead of the insertional sites of posterior mitral leaflets.^[47]

Assessment of fluoroscopic angles

During the TMVR procedure, the cardiologists need fluoroscopic angle that is perpendicular to the mitral annulus for coaxial deployment of the prosthesis. Any obliquity may lead to increased fluoroscopic times and higher radiation dose to the patient. CT imaging can be used to generate this patient-specific angle. CT postprocessing software generates a coplanar curve specific to the patient which provides tangential view of the valve allowing coaxial deployment.

Conclusion

Both TAVR and TAVR have rapidly gained favor in the treatment of symptomatic aortic valve stenosis and MR, respectively, in the nonsurgical patient cohort. The success of these procedures is greatly dependent on a thorough preprocedural imaging screening. MDCT is the cornerstone in the selection of eligible patients, in choosing the appropriate prosthesis and size, and in mapping the safest access route for the intervention.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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